🧹 Fix silent error handling in writer.ts

[KevinArce]

This change addresses a code health issue where file I/O errors in src/writer.ts were silently swallowed.

Changes:

• Updated WriteResult interface to include an optional error property of type Error.
• Modified appendToEnv, syncExampleFile, and generateTypeDefinitions in src/writer.ts to catch errors and return them in the WriteResult object.
• Updated src/ui.ts to check for and log the error message if appendToEnv fails.
• Added a new test case in tests/writer.test.ts to verify that write failures (simulated by creating a directory at the target path) result in success: false and a populated error property.

Verification:

• Verified changes by running bun test tests/writer.test.ts. The new test case should return error when writing fails passed.
• Other tests in tests/writer.test.ts also passed, confirming no regression in existing functionality.

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PR created automatically by Jules for task 15399871764976004732 started by @KevinArce

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